1. Field of the Invention
This invention relates to bending and tempering by contact of glass sheets, particularly for the production of glazings intended, for example, for motor vehicles, such glazing requiring a very great shape precision, fragmentation (upon breaking) according to safety standards, and an optical quality satisfying the strictest standards.
2. Description of the Related Art
It is known, for example, from patent application EP-A-277074 to simultaneously bend and temper glass sheets which have previously been heated beyond their plastic deformation temperature, by bringing them between two cooling and compression plates whose form corresponds to the curve that it is desired to impart to the glass sheet. The glass sheets are pressed between the plates until their temperature is low enough for their shape to remain fixed. Such a process is particularly advantageous for the glazings of thin glass, for example of less than 3 mm of thickness, in which it is difficult to bring them after bending to a high enough temperature for a heat tempering; this is because of the quickness of their cooling in air as soon as the glazings of thin glass come out of the heating furnace.
Another reason for developing such processes of bending-tempering by contact is the need for a perfect control of the curve of a glazing, particularly the elimination of the so-called defects of double-bending, i.e., undesired curves, due to the effects of gravity when the glass sheet is not supported at every point of its lower face.
The present inventors have observed a slight divergence of the form of the glass sheet relative to that of the cooling and compression plates, whose origin is partially due to the incomplete relaxation of the bending stresses. These bending stresses are maximal in the vicinity of the surfaces of the glass, i.e., they are maximal at the locations cooled most quickly and therefore for which the period during which the relaxation of the stresses can actually occur is minimal. Further, the nonrelaxation of the stresses can cause localized tensions which are greater than the resistance of the glass, and so the breaking of the glazing.
It is possible to extend this period during which this relaxation can occur by beginning with glass sheets whose initial temperature is higher and which therefore undergo a cooling of greater intensity. However, there exists in terms of optical quality a limit to the initial heating of the glass sheet. If the glass sheets are transported flat on rollers--which is the mode of transport which is the simplest, the most economical and for the most part the least likely to cause marks or smears in the enamel--glass which is too hot has a slight tendency to sag between the rollers, which creates defects of the "corrugated sheet" type whose trace persists even after pressing during the stage of bending-tempering by contact.
Further, with bending-tempering by contact, it is practically impossible to extend the shaping phase by, for example, gradually folding the bending forms, because the shaping time must be considered if the glass sheet simultaneously undergoes an intense cooling by the cooling and compression plates. The use of suitable buffer materials supplies a solution to this problem, but only with respect to the usual pressing speeds for tempering by contact. Actually, if the assigned shaping time is too long, a breakage occurs at the end of the shaping, because the glass is then too cold to support the deformation rate. This limiting of the shaping time causes a limiting of the curves able to be imparted by this process. Moreover, extending the shaping phase reduces the operating speed of the production line, which is always a defect for an industrial use.
On the other hand, many processes of production of bent and tempered glazings are known in which these two operations are performed in separate times and places, particularly processes according to which the glass sheets are heated in a horizontal position in a furnace through which they are conveyed, for example by a conveyor formed by rollers, then are lifted above the conveyor by mechanical or pneumatic means--particularly by suction or by blowing hot air--applied against a flat upper element or one constituting a bending form, and then released on a lower element, for example a ring open at its center, which guides the glass sheet to a heat tempering device which blows cold air via tempering boxes. Depending on the case, the shaping of the glass sheet is performed exclusively or partially by the lower element, or it is already finished when the glass sheet is deposited on the lower element.
Processes also are known which do not use a lower element --or which use one only for the bending stage--and according to which the transfer of the bent glass sheet to the tempering device is performed directly on the upper form which then is moving, or on a conveyor formed from an optionally curved roller bed. To these processes of bending-tempering using an upper element and the vertical movement of the glass sheet, it also is necessary to add the bending processes according to which the glass sheet is advanced on a shaping bed consisting, for example, of markers or rods mounted straight on a curved path. All the processes mentioned above are well-known in the art and examples of them can be found in patent publications US-A3,527,589, EP-A-3391, EP-A-5306, FR-A-2085464, FR-A-2312463, FR-A-2442219, FR-A-2549464, FR-A-2549465, FR-A-2554436, FR-A-2567508, FR-A-2596750, FR-A-2596751.
These processes make it possible to reach very high speeds even for relatively complex glazing forms. However, if priority is always given to the optical quality, it is practically impossible to obtain perfect conformity with the form and particularly to prevent the double-bending defect. It is necessary to note that the divergences of the form are all the more appreciable when the thickness of the glass sheet is slight, which explains the increased advantage of processes of bending-tempering by contact for thin glass.